Panic!! A mysterious electrical problem

[HansH]

Fault message: battery charge or electrical system on my C5 (X7) 2.7 HDI V6 from 2008

I got this malfunction driving home with another 70 kilometers to go. Coming home, the car immediately went into Eco mode and the battery was completely flat.
Conclusion: Alternator faulty ...

I have replaced the alternator for a revised exchange alternator. Which was a huge pain in the arse; Wheel off, all the tupperware off and almost no room to tinker while you are lying on your back under the car, grrr!

Anyway; success! put things together again and everything worked as it should, loading voltage neatly towards 14 volts.
The old alternator had a problem with the pulley, which moved free without taking the coils with them. So truly broken.

But then! Made a ride yesterday and again that error message
Charging voltage measured and it fluctuates quite between 13 and 13.5 volts. Car deposited and let cool, restart and no more malfunction, can drive for about 20 kilometers. Stand still for a moment, start again and after about 10 kilometers again the battery/loading voltage fault message.

I have put the battery charger on the car on the poles under the hood (the battery itself is in the back of the cargo space)
What was striking is that he does load but with just some 13 volts. If I take the battery loose and connect the same charger directly on the battery, it loads with almost 14 volts.

I have two suspicions:

1) With the fluctuating loading voltage while driving, the voltage sometimes dives under the 13V so that the malfunction is generated.

2) The problem is in the installation, presumably in that red box on the battery + pool that must control the loading voltage.

I do not understand that system and have no idea where to start with failure search in that box.

Who can save me?!
The panic I experience is that I have to drive 300km on Friday, mainly motorway, with two large dogs in the back. I have two full spare batteries, but even then I think it is a risk.

Thanks for any advice!

[GiveMeABreak]

Hard to suggest anything without a diagnostic. If you can , provide the fault codes. I would of suggested that the battery was discharged and with the drain on it after replacing the alternator, that it wasn't getting charged sufficiently. I know you say you have several spare batteries, but remember a battery 5 years and older needs replacing, as their capacity becomes much diminished with the demands from the electrical systems.

[HansH]

Thanks for the reply!

I’m charging my old Windows laptop atm to do some diagnostics with the diagbox.
It’s raining cats and dogs right now so that’ll be a job for tomorrow.
I’ll also measure the voltage on the alternator directly to make sure that it’s willing to charge.
Then I will check the wiring, especially the negatives throughout the car!
Maybe check the fuses below the BSM just to be sure but i doubt that the problem lies there…

Will get back here once i have more information!!

Cheers so far